1. Field of the Invention
The present invention relates to an improvement in a focusing state detection apparatus which detects a focusing state by a relative positional relationship of light intensity distribution of two images of an object.
2. Related Background Art
In a prior art focus state detection apparatus for a camera, an exit pupil of an imaging lens is divided into two pupil areas and relative positional displacement of two light intensity distributions of two images formed by light fluxes transmitted through the pupil areas are observed to determine an in-focus state. For example, Japanese Patent Application Laid-Open Nos. 118019/1980 and 155331/1980 disclose a secondary focusing system in which a spatial image formed on a predetermined focus plane (corresponding to a film plane) by two secondary focusing optical systems is guided to two sensor planes so that relative positional displacement of the two images is detected.
The secondary focusing type focusing state detection apparatus is shown in FIG. 3. A field lens 3 is arranged coaxially with an optical axis 2 of an imaging lens 1 whose focusing state is to be detected. Two secondary focusing lenses 4a and 4b are arranged behind the field lens 3 symmetrically with respect to the optical axis 2. Sensor arrays (Photo-electric conversion element arrays) 5a and 5b are arranged behind the lenses 4a and 4b. Diaphragms 6a and 6b are arranged in the vicinity of the secondary focusing lenses 4a and 4b. The field lens 3 essentially focuses an exit pupil of the imaging lens 1 onto pupil planes of the two secondary focusing lenses 4a and 4b. As a result, light fluxes applied to the secondary focusing lenses 4a and 4b correspond to those light fluxes which are emitted from non-overlapping equispace areas on the exit pupil plane of the imaging lens 1, corresponding to the secondary focusing lenses 4a and 4b. When a spatial image formed in a vicinity of the field lens 3 is refocused on the planes of the arrays 5a and 5b by the secondary focusing lenses 4a and 4b, the positions of the two light intensity distributions of the two images on the sensor arrays 5a and 5b change in accordance with the displacement of the spatial image along the optical axis. FIG. 5 shows it. In FIG. 4A which shows an in-focus state, the two light intensity distributions are positioned at the centers of the sensor arrays 5a and 5b, in FIG. 4B which shows a near-focus state, the two light intensity distributions are moved away from the optical axis 2, and in FIG. 4C which shows a far-focus state, the two light intensity distributions are moved toward the optical axis 2. These light intensity distributions are photo-electrically converted and the converted electrical signal is processed to detect a relative positional deviation of the two light intensity distributions. In this manner, the focusing state of the imaging lens 1 can be detected.
Methods for processing the photo-electrical converted signal from the sensor arrarys 5a and 5b are disclosed in Japanese Patent application Laid-Open No. 142306/1983 and U.S. Pat. No. 4,333,007. Specifically, the following formula is operated for k.sub.1 .ltoreq.k.ltoreq.k.sub.2. ##EQU1## where N is the number of sensors of the sensor array 5a or 5b, A(i) and B(i) are image signals from the i-th elements of the sensor arrays 5a and 5b, and M is the number of pixels processed (M=N-k.vertline.-1) A(i).quadrature.B(j) is an operator for A(i) and B(j). For example, EQU A(i).quadrature.B(j)=.vertline.A(i)-B(j).uparw. (2) EQU A(i).quadrature.B(j)=.vertline.A(i)-B(j).uparw..sup.n ( 3) EQU A(i).quadrature.B(j)=max.vertline.A(i), B(j).uparw. (4) EQU A(i).quadrature.B(j)=min.vertline.A(i) B(j).uparw. (5)
The formula (2) represents an absolute value of a difference between A(i) and B(i), the formula (3) represents accumulated product, the formula (4) represents a larger one of A(i) and B(j), and the formula (5) represents a smaller one. By the above definition, V.sub.1 (k) and V.sub.2 (k) can be considered as correlation amounts in a broad sense from the formula (1), V.sub.1 (k) represents the correlation amount at a displacement (k-1), and V.sub.2 (k) represents the correlation amount at a displacement (k+1). Accordingly, an evaluation amount V(k) which is the difference between V.sub.1 (k) and V.sub.2 (k) represents a change of correlation amount of the image signals A(i) and B(i) at a relative displacement k. Since a change is zero at the peak of the correlation amount, it is assumed that the peak of the correlation amount exists in a section [k, k+1] represented by EQU V(k).multidot.V(k+1)&lt;0 (6)
and V(k) and V(k+1) are interpolated to detect the deviations of the image signals A(i) and B(i). FIG. 6 shows the light intensity distribution signals A(i) and B(i) for the two images formed when the number of sensors is 16 (N=16). There is a deviation of P. FIG. 7 shows the evaluation amount V(k) of the formula (2) when the relative displacement k is changed within a range of -N/2.ltoreq.k.ltoreq.N/2. As described above, V(k) and V(k+1) which meet V(k).multidot.V(k+1)&lt;0 are linearly interporated to detect the deviation P. FIG. 7 shows a relationship between the image signals A(i) and B(i) when the evaluation amount V(k) is calculated while the relative displacement k changes in a range of -3.ltoreq.k.ltoreq.3. Hatched areas show the sensors which are subject of the correlation.
The processing time of the evaluation amount V(k) significantly varies with a range of processing of the relative displacement k. Accordingly, it is preferable to limit the processing range to a narrow one. However, if the range is too narrow, the deviation of the two images when the imaging lens is in a large defocus state may depart from the processing range of the relative displacement k so that exact focusing state is not detected. Accordingly, a lower limit k.sub.1 and an upper limit k.sub.2 of the processing range are frequently set to k.sub.1 =-N/2 and k.sub.2 =N/2, where N is the number of sensors of the sensor array. However, where the processing range for the relative displacement k is preset, the processing may be done over unnecessary range if an imaging lens has a small defocusing state like a wide angle lens. In other words, excess processing is performed and unnecessary processing time is spent for the focusing state detection.
When the deviation P is calculated, contrasts of the two images are usably calculated. If the contrast is lower than a predetermined level, it is determined that the reliability of the resulting deviation P is low and the foucsing state detection is disabled. The disabling operation includes a so-called searching operation, in which the imaging lens is driven by a predetermined distance or continuously while expecting the increase of the contrast necessary for the focusing state detection. However, in the imaging lens having a relatively small maximum defocus amount such as a wide angle lens, the increase of the contrast by the searching operation is not expected because the defocus amount is inherently small.
On the other hand, if the imaging lens has a large maximum defocus amount such as a telescopic lens and the lens is in a large defocus state, a probability that the deviation P of the two images departs from the relative displacement k is high and the focusing state may not be detected.
In the prior art apparatus, since the focusing state detection operation takes a long processing time, high speed photographing cannot be attained particularly in a continuous photographing mode.